1. Field of the Invention
Aspects of the present invention relate to a lithium secondary battery, and more particularly, to positive electrode active materials for a lithium secondary battery, a method of preparing the same, and a lithium secondary battery comprising the same.
2. Description of the Related Art
Lithium secondary batteries repetitively shift lithium ions between positive and negative electrodes. Lithium secondary batteries use materials that intercalate and deintercalate lithium ions, as positive and negative active materials. Typically, carbon-based, or metallic (including metal oxides) carbon composite materials, are used as the negative active materials, and lithium-metal oxides are used as the positive active materials.
Metallic cobalt is widely used as a positive electrode active material. In order to improve other characteristics, or to address resource scarcities, other metals, such as Ni, Mn, etc., (particularly transition metals) have been used as active materials. Although cobalt is largely employed as a positive electrode active material, lithium metal oxides, which are metal composites comprising lithium and other metals, are also used.
The metal composites may be crystalline metal composites, such as LiMO2, LiM2O4, etc., wherein M may be represented by the formula: Ni(1-x-y)CoxMny (wherein x and y are positive numbers less than 1, and 1−x−y is less than or equal to 1).
Lithium cobaltate (LiCoO2) has stable charge/discharge characteristics, excellent electron conductivity, and flat discharge voltage characteristics. However, the development of other materials is desirable, due to the scarcity, expense, and toxicity of lithium coboltate.
Lithium nickelate (LiNiO2) has a layered structure similar to lithium cobaltate, has a large discharge capacity, but is difficult to form into a pure layered structure. Lithium nickelate is converted into LixNi1-xO, which has a rock salt-type structure, due to reactive Ni4+ ions produced during charging, while emitting excess oxygen, which leads to reduced cycle life and thermal instability. Nickel-cobalt based positive electrode active materials that substitute some of the nickel with cobalt, for example LiNi1-xCoxO2 (where x=0.1˜0.3), exhibit excellent charge/discharge and cycle life characteristics, but still have problems with thermal instability.
On the other hand, lithium metal oxides in positive electrode active materials may form a crystalline phase, or a mixed crystalline and amorphous phase, according to the formation method thereof. Even in the crystalline phase, lithium metal oxides may form a mixture of two different crystal structures. Such differences in crystal structure lead to differences in reactivity between active materials and ambient materials, during battery reactions.
If the positive electrode active materials participate in different side reactions with electrolyte components, in accordance with the different crystalline structures, by-product layers, such as a solid electrolyte interface (SEI) layer, can be formed on the surfaces of the positive active materials. The by-product layers have different ion conductivities from an electrolyte, thereby leading to a variation in cell resistance. An increase in the cell resistance results in a decreased in cell efficiency, degraded cell function, and a reduction in cycle life, due to the generation of heat.
In a high temperature storage test, nickel-cobalt based lithium ion batteries are stored at 60° C. for 50 days, at a 4.2 volt full charge. The batteries' resistances are then compared before and after the high-temperature storage. Such comparisons show increases in resistance of more than around 150% after the test.